The ability to communicate using a handheld communications device, e.g., a portable telephone, regardless of one's location in a wide area is of great value. The value of such a device is important to military applications as well as in the case of conventional consumer based applications.
Terrestrial base stations have been installed at various earth based locations to support voice and/or data services. Such base stations normally have a coverage area of a few miles at most. Accordingly, the distance between a conventional cell phone and a base station during use is normally only a few miles. Given the relatively small distance between a cell phone and a terrestrial base station during normal use, a hand held cell phone normally has sufficient power to transmit to the base station, e.g., on an uplink, using bandwidth that is relatively wide and, in many cases, capable of supporting relatively high data rates.
In the case of one known system based on the use of terrestrial base stations, a plurality of OFDM tones, e.g., in some cases 7 or more tones, are used in parallel by a wireless terminal to transmit user data to the base stations. In the known system, user data to be communicated via an uplink and control signals to be communicated via an uplink are normally coded separately. In the known system, a wireless terminal may be assigned a dedicated tone for uplink control signaling with uplink traffic segments which correspond to tones being assigned in response to one or more uplink requests transmitted to the terrestrial base station. In the known system uplink traffic channel segment assignment information is broadcast to the wireless terminals which monitor assignment signals that may indicate assignment of uplink traffic channel segments in response to a transmitted request. On a recurring basis, the base station of the known system also broadcasts signals which can be used for timing synchronization with the timing synchronization signals, referred to as beacon signals, recurring over a time period sometimes referred to as a beacon slot.
While terrestrial base stations are useful in areas where the population is sufficient to justify the cost of a terrestrial base station, in many locations on the planet there is insufficient commercial justification to deploy a base station and/or due to geographic issues it is impractical to deploy a permanent terrestrial base station. For example, in physically inhospitable areas such as the open ocean, dessert regions and/or regions which are covered by ice sheets it may be difficult or impractical to deploy and maintain a terrestrial base station.
The lack of base stations in some geographic regions leads to “dead zones” in which is not possible to communicate using a cell phone. In order to try and eliminate the number of areas where cell phone coverage is missing, companies are likely to continue to deploy new base stations but, for the reasons discussed above, for the foreseeable future there are likely to remain large areas of the planet where cell phone coverage from terrestrial base stations can not be obtained.
An alternative to terrestrial base stations is to use satellites as base stations. Satellite base stations are extremely costly to deploy given the cost of launching satellites. In addition, there is limited space above the planet in which geostationary satellites can be placed. While satellites in geostationary orbit have the advantage of being in a fixed position relative to the earth, lower earth orbiting satellites can also be deployed but such satellites remain costly to deploy and will remain in orbit for a shorter period of time due to their initially lower orbit than a geostationary satellite. The distance from the surface of the earth where a mobile phone may be located and geostationary orbit is considerable, e.g., approximately 22,226 miles although some estimates suggest that 22,300 miles is a better estimate. To put this in perspective, the diameter of the Earth is approximately 7,926 miles. Unfortunately, the distances which signals must travel in the case of satellite base stations is considerable longer than the distance signals normally travel to reach a conventional terrestrial base station which is usually a few miles at most.
As can be appreciated, given the distance to geostationary orbit, it is often necessary to transmit signals to satellites at higher power level than is required to transmit signals to terrestrial base stations. As a result, most satellite phones normally are relatively large and bulky compared to conventional cell phones due to the size of the batteries, power amplifiers and other circuitry which has been used to implement cell phones. The need for a relatively large, and therefore often bulky, power amplifier results, in part, from the fact that many conventional communications systems have a less than ideal peak to average power ratio. The relatively large peak to average power ratio requires that a larger amplifier be included to support peak power output than could be used in the case of the same average power output, but where the peak to average power ratio is lower.
Given the large distance to a satellite base station and/or comparatively large cell size, as compared to a terrestrial base station, uplink timing synchronization used for terrestrial base stations which use OFDM signals in the uplink may not be sufficient to achieve adequate uplink symbol timing synchronization when communicating with a satellite base station. Accordingly, there is a need for improved methods of supporting OFDM uplink signaling including improved timing synchronization methods and/or apparatus which can be used with long round trip delays.